1. Field of the Invention
The present invention relates generally to a process for manufacturing a lead frame for a semiconductor package, and, more particularly, to a process for manufacturing a lead frame for a semiconductor package, of which the back surface of the pad thereof is coated with a polyimide film in order to prevent, after an electronic device containing the lead frame is encapsulated by a molding compound, delamination (or separation) of the molding compound from the pad.
2. Description of the Prior Arts
Lead frames, are used for packaging semiconductor devices such as transistors, or integrated circuits, which are mounted onto the pads of respective lead frames, and then encapsulated. A lead frame includes a lead frame pad onto which a semiconductor chip(s) is(are) mounted, leads electrically connected to the chips through wires, and dambars for preventing flowing out of molding resin during encapsulation.
A lead frame may further have a plurality of dimples formed on its "back surface or inactive surface" (the surface opposite to the surface onto which semiconductor chip is mounted) of its lead frame pad to improve the bonding force between the die pad and the molding resin, in order to prevent separation (or delamination) of the resin from the pad after completing packaging, due to difference of thermal expansion coefficient between the resin and the pad. Lead frames formed with dimples have an important role in improving reliability of ultraslim semiconductor packages such as TSOP (Thin Small Outline) or SOJ (Small Outline "J" Bending) Packages.
A lead frame also may be coated with a polymer film, for example a polyimide film on the back surface of the pad, to improve the bonding force between the lead frame pad and the molding resin. The film is bonded to back surface of pad by adhesive, and may have a plurality of dimples, usually formed by a stamping method.
Since ultraslim semiconductor packages such as TSOP or SOJ Packages are very thin, they are fragile. Further, moisture from external environments may penetrate into the package, resulting in separation of the molding resin from the pad.
FIG. 1 is a schematic plan view of a conventional lead frame for a semiconductor package.
FIG. 2A is a bottom palan view of a prior art lead frame pad, of which the back surface is provided with circular dimples; and FIG. 2B is a cross; sectional view along line A--A of FIG. 2A.
FIG. 3A is a bottom view of a prior art lead frame pad, of which the back surface is provided with diamond shaped dimples; and FIG. 3B is a cross; sectional view along line B--B of FIG. 3A.
With now reference to FIGS. 1 through 3B, lead frame (100) comprises a lead frame pad (50) onto which a semiconductor chip (not shown) is mounted by an adhesive (not shown), inner leads (30) of the lead frame being electrically connected to the bonding pad (not shown) formed on the chip, through wires (not shown), outer leads (40), which are integrated with respective ones of the inner leads and are electrically connected to respective external terminals (not shown), dam bars (20) for preventing flowing of the molding resin during encapsulation of the chip, wires and lead frame pad by the resin, side rails (10) formed at the top and bottom ends of the lead frame (100), and index holes (12), which are formed within the side rail (10), for appropriately transferring the lead frame by a transferring means (not shown).
The dimples (155, 255) may be formed on back surface (150) of the lead frame pad by using an etching method, or formed on polyimide film (250) adhered to the back surface of the lead frame pad by using a stamping method. The etching method forms various shaped dimples including round dimples (155), while the stamping method forms mainly diamond shaped (or chamfered) dimples (255).
The etching method has the advantages that it can form round dimples, which provides highly reliable packages, and that it can be applied to manufacture of TSOP or SOJ Packages, which requires high reliability. Further, it may forms various shapes of dimples, including round dimples.
However, it has the drawbacks that, since it requires etching of the lead frame pad itself, the production cost is high, while productivity is low.
By contrast, the stamping method is advantageous over the etching method in that the productivity is high and the production cost is low. However, it cannot form round dimples. Further, diamond shaped dimples, which are formed by the stamping method, show no effect in improving the bonding force between the pad and the resin, in case of ultra-slim packages such as TSOP or SOJ Packages.
FIG. 4A is a bottom plan view of a conventional lead frame, of which the pad is coated with polyimide film on its back surface; and FIG. 4B is a cross; sectional view along line C--C of FIG. 4A.
With now reference with FIG. 4, lead frame pad (350) is coated with a layer of polyimide film (310) on its back surface, the film being held in place by a layer of adhesive (320). In this prior art construction, the adhesive layer (320) serves as a path for the penetration of moisture from the external environment, resulting in a peeling off (or delamination) of the molding resin (310) from the pad (350).
Accordingly, there has been a need to provide lead frames without disadvantages which have been described above.